(1) Field of the Invention
The present invention relates to a propelling apparatus for a vehicle drivable by four wheels, comprising a hydraulic pump or pumps driven by an engine, and hydraulic motors operable by pressure oil supplied from the hydraulic pump or pumps to drive ground wheels.
(2) Description of the Prior Art
A propelling apparatus as constructed above is disclosed in Japanese Utility Model Publication Kokai No. 56-62283, for example. This apparatus comprises two pumps driven by an engine, a hydraulic motor for driving front wheels, and a hydraulic motor for driving rear wheels.
Some four wheel drive vehicles have a propelling system switchable between a two wheel drive mode for driving either the front wheels or the rear wheels and a four wheel drive mode for driving both the front and rear wheels. For simplicity and low cost, such a propelling system may include a single hydraulic pump for driving both the front and rear wheels.
More particularly, the propelling system comprises a single hydraulic pump, and a shutoff valve or switch valve mounted on an oil line extending to a hydraulic motor for driving the front or rear wheels. For four wheel drive, the shutoff valve is set to an open position (communicating position) to drive both the hydraulic motors. For two wheel drive, the shutoff valve is set to a closed position (shutting position) to drive only one of the hydraulic motors. In the construction in which the two wheel drive mode is established by operating the shutoff valve, the hydraulic motors remain interlocked with the ground wheels even when the shutoff valve is set to the closed position. Thus, in the two wheel drive mode, pressure oil is not supplied to one of the hydraulic motors, and this motor itself does not output power but receives torque transmitted from the ground wheels.
It means that, in the two wheel drive mode, the hydraulic motor to which pressure oil is not supplied is rotated by the torque transmitted from the ground wheels to act like a hydraulic pump for pumping the pressure oil. This not only results in an energy loss but tends to damage the hydraulic motor.